While pharmacological methods have been traditionally used to define receptor types and subtypes, receptor cloning experiments have often led to the discovery of novel receptor types and subtypes within many receptor families. Following the cloning of the melanocyte stimulating hormone (MSH) gene [see Science 257:543 (1992)] and the adrenocorticotropic hormone (ACTH) receptor gene [see FEBS Lett. 309:417 (1992)], for example, three unique yet related genes were identified that also encoded functional, high affinity receptors for the MSH and ACTH peptides [see PNAS USA 90:8856 (1993); J. Biol. Chem. 268:8246 (1993); J. Biol. Chem. 268:15174 (1993);
Biochem. Biophys. Res. Comm. 200:1214 (1994); Biochem. Biophys. Res. Comm. 195:866 (1993); Biochem. J. 299:367 (1994); Biochem. 33:4543 (1994); Mol. Endo. 8:1298 (1994); J. Mol. Endochrinol. 12:203 (1994); and Biochem. Biophys.
Res. Comm. 200:1007 (1994)]. Named by number in the order of their discovery, the melanocortin-1 receptor gene has been found thus far to be expressed primarily in the epidermal tissues; melanocortin-3, melanocortin-4, and melanocortin-5 receptor genes have been found thus far to be expressed primarily in the hypothalamus, mid-brain and brainstem (MC3-R, and MC4-R), or in a wide distribution of peripheral tissues (MC5-R).
The melanocortin peptides have been reported to have a wide variety of biological activities outside of their effects upon pigmentation and steroidogenesis, known to be mediated by the MSH and ACTH receptors. However, given the complexity of possible sites of expression of the MC3, MC4 and MC5 receptors, it has not been possible to unambiguously identify any simple correlation between these receptors and the reported biological activities of their ligands. Consequently, potent and specific agonists and antagonists would be extremely valuable tools for determining the physiological roles of the MC3, MC4 and MC5 receptors; the roles of the MC1 (pigment cell receptor) and MC2 (primary receptor for ACTH in the adrenal gland) receptors are fairly well-documented.
While prior structure-function analyses have been reported in the past on the affinity and potency of the .alpha.-MSH peptide at the MSH receptor site [for reviews see Peptide Protein Rev. 3:1 (1984), The Melanotropic Peptides, Vol. I, II, and III (CRC Press) (1988)], only a few relatively weak antagonists have resulted from these studies [see Int. J. Peptide Protein Res. 35:228 (1990); Peptides 11:351 (1990); and Peptide Res 3:140 (1989)].
Accordingly there is still a need to provide for potent and specific antagonists that will allow for the determination of the physiological roles of these melanocortin receptors.